Plastic castings molds

ABSTRACT

The invention is concerned with the problem that with known plastic casting moulds, especially those of polypropylene, the lenses produced with these moulds have a slippery surface. The invention solves this problem through the use of polymers which are notable for their very low oxygen permeability.

The invention relates to a process for the preparation of mouldings,especially optical lenses and in particular contact lenses, to acorresponding device for the preparation of mouldings, and to themouldings that have been prepared or are obtainable by the process orusing the device, especially optical lenses and in particular contactlenses, in accordance with the preamble of each independent patentclaim.

Contact lenses, which are to be manufactured economically in large unitnumbers, are preferably manufactured by the so-called mould orfull-mould process. In these processes, the lenses are manufactured intotheir final shape between two moulds, so that there is no need tosubsequently finish the surfaces of the lenses, nor to finish the edges.Such moulds consist of a female mould half and a male mould half, thecavity being formed between the two mould halves defining the shape ofthe moulding. Mould processes are described for example in PCTapplication no. WO/87/04390 or in European patent application EP-A-0 367513.

In these known mould processes, the geometry of the contact lenses to bemanufactured is defined by the mould cavity. The edge of the contactlens is likewise formed by the mould which normally consists of twomould halves. The geometry of the edge is defined by the contour of thetwo mould halves in the area in which they touch one another.

To prepare a contact lens, first of all a certain amount of a flowablestarting material is placed in the female mould half. Afterwards, themould is closed by placing the male mould half thereon. Normally, asurplus of starting material is used, so that, when the mould is closed,the excess amount is expelled into an overflow area outwardly adjacentto the mould cavity. The subsequent polymerisation or crosslinking ofthe starting material takes place by radiation with UV light, or by heataction, or by another non-thermal method. Both the starting material inthe mould cavity and the excess material in the overflow area arethereby hardened. In order to obtain error-free separation of thecontact lens from the excess material, a good seal or expulsion of theexcess material must be achieved in the contact zone of the two mouldhalves. Only in this way can error-free contact lens edges be obtained.

The contact lenses produced in this manner are moulded parts havinglittle mechanical stability and a water content of more than 60% byweight. After manufacture, the lens is inspected, then packed andsubjected to heat sterilisation at 121° C. in an autoclave.

The materials used for these moulds are preferably plastics, e.g.polypropylene. The moulds are produced by injection moulding and areonly used once. This is because, among other things, the moulds arepartially contaminated by the surplus material, are damaged when thecontact lens is separated or are irreversibly deformed in partial areaswhen the mould is closed. In particular, because of the qualityrequirements of the contact lenses edges, the moulds are only used once,since a certain amount of deformation of the moulds at the area of theiredge cannot be excluded with certainty.

In U.S. Pat. No. 5,508,317, a new contact lens material is described,which represents an important improvement in the chemistry ofpolymerisable starting materials for the manufacture of contact lenses.The patent discloses a water-soluble composition of a prepolymer, whichis filled into the mould cavity and then crosslinked photochemically.Since the prepolymer has several crosslinkable groups, the crosslinkingis characterised by its high quality, so that a finished lens of opticalquality can be produced within a few seconds, without the necessity forsubsequent extraction or reworking steps. Owing to the improvedchemistry of the starting material as illustrated in the patent, contactlenses can be produced at considerably lower cost, so that in this wayit is possible to produce disposable lenses that are used only once.

EP-A-0 637 490 describes a process by means of which a furtherimprovement may be obtained in the preparation process of contact lenseswith the prepolymer described in U.S. Pat. No. 5,508,317. Here, thematerial is filled into a mould comprising two halves, whereby the twomould halves do not touch, but a thin circular gap is located betweenthem. The gap is linked to the mould cavity, so that surplus lensmaterial can flow away into the gap. Crosslinking of the prepolymertakes place by radiation especially with UV light, whereby radiation isrestricted to the mould cavity by a chromium mask. In this way, only thematerial in the mould cavity is crosslinked, so that there is highreproducibility of the edges of the lens without closing the two mouldhalves.

In this process, instead of the polypropylene moulds that may be usedonly once, reusable quartz/glass moulds are used. Because of thewater-soluble basic chemistry, after a lens has been produced, theuncrosslinked prepolymer and other residues can be removed from themoulds rapidly and effectively and dried in the air. In addition, quartzis notable for its good UV permeability and is very hard and refractory.

When using a prefunctionalised PVA (polyvinyl alcohol) polymer as lensmaterial, moreover, important material properties of quartz castingmoulds are the excellent replication of the surface geometry, as well asthe transparency of the material, so that it is possible to providevisual control of the lens in the mould.

However, moulds made from quartz or glass are very expensive to produce,so that owing to the high costs, the moulds ought to have quite longservice life in order to ensure that the process is economical.Therefore, for economic reasons, only a limited number of variants canbe realised, for example in respect of the dioptre number.

In order to solve this problem, the intended objective is to replace atleast one of the two mould halves, especially the female mould half,with a mould half consisting of plastic, and thus to employ one mouldhalf made of plastic in combination with one mould half made of quartzor glass.

In the plastic moulds, especially polypropylene moulds, which werepreviously known in the prior art, there was however the problem thatthe lenses produced with these moulds had a slippery surface. This isbecause the slipperiness is caused by oxygen, which leads to aninhibition of the crosslinking reaction at the surface of the lens, thisbecoming apparent macroscopically in a “slipperiness” of the lens. It isassumed that oxygen, which is already present at the surface of thecasting mould or migrates to the surface of the mould during thepolymerisation process, inhibits polymerisation of the lens material atthe lens surface. Thus, the interface between the mould and the lensmaterial appears to be crucial.

In order to avoid this problem, it is known from EP-A-0 687 550 that theplastic mould halves can be degassed and then transported and filled ina nitrogen atmosphere. This however requires a technical process that isvery complex and in addition is very cost-intensive. WO-A-96/11782describes a process operating under an inert gas atmosphere and/or oneusing moulds in which the oxygen contained therein has been drawn offcompletely by means of treatment with a vacuum or with an inert gas,thus producing “non-slippery” lenses. It also describes that the oxygenin the material from which the lenses are produced is completely drawnoff prior to lens manufacture. However, the cost of these additionalprocess steps is very high and the process steps are verytime-consuming.

It is the aim of the present invention to further develop a process anda device of the generic kind and to improve them in such a way that itis possible to use mould halves of plastic in conjunction with mouldhalves of quartz or glass, without the above-mentioned difficulties andproblems arising. In particular, the plastic mould halves should bereusable, and any burr or web formation on the finished contact lensesshould be avoided, so that the rejection rate of the contact lenses isvery low. In addition, optical monitoring of the lens through thecasting mould should be possible.

The invention solves the problem by means of the features indicated inclaim 1 and claim 11. As far as further essential developments of theprocess according to the invention and of the device according to theinvention are concerned, reference is made to the dependent claims.

Through the choice of polymers for the production of plastic mouldhalves, which have very low oxygen permeability, the oxygen is preventedto a great extent from reaching the cavity during polymerisation, andthus prevented from making contact with the starting material for thelenses. The idea behind this is that the oxygen which impairspolymerisation or crosslinking of the lens surface originates from thecasting mould with which the lens material comes into contact. It couldbe shown that when using polymers for the mould, which have only slightoxygen permeability, lenses having no surface slipperiness can beproduced. It is thus possible to manufacture lenses in an atmosphericenvironment whilst simultaneously using plastic mould halves. Since itis not necessary to produce the lenses in a nitrogen atmosphere,production costs may be substantially reduced.

Further aspects and advantages of the process according to the inventionand of the device according to the invention will be seen from thedescription that follows, in conjunction with the drawings. In thedrawing,

FIG. 1 shows a section through an embodiment of a casting mouldaccording to the invention in closed state;

FIG. 2 shows a section through a second embodiment of a casting mouldaccording to the invention in closed state;

The device shown in FIG. 1 is designed for the manufacture of contactlenses from a liquid starting material which may be polymerised orcrosslinked by UV radiation. It comprises a mould 1 and an energy source2 a, here a UV light source, as well as means 2 b for directing theenergy provided by the energy source 2 a to the mould in the form of anessentially parallel beam. Of course, the energy source 2 a and means 2b can also be combined to form a single unit.

The mould consists of two mould halves 11 and 12, each having a curvedmould surface 13 and 14 which together define a mould cavity 15, whichin turn determines the shape of the contact lens to be manufactured. Themould surface 13 of the upper mould half 11 in the drawing is convex anddetermines the rear and basic face of the contact lens with theconnected edge area; this mould half is normally called the father mouldhalf. Conversely, the mould surface 14 of the other mould half, which iscorrespondingly called the mother mould half, is concave and determinesthe front face of the contact lens to be manufactured, likewise with theconnected edge area.

The mould cavity 15 is not completely and tightly closed, but in theembodiment illustrated is open around its peripheral edge which definesthe edge of the contact lens to be manufactured, and is linked to arelatively narrow annular gap 16. The annular gap 16 is limited orformed by a flat mould wall 17 and 18 on each of the father mould half11 and the mother mould half 12. In order to prevent complete closure ofthe mould, spacers, for example in the form of several bolts 19 a or 19b, are provided on the mother mould 12, and these interact with a collaror flange 20 of the father mould 11 and keep the two mould halves atsuch a distance apart that the said annular gap 16 results. As isindicated symbolically in FIG. 1 by the right-hand spacer bolt 19 b witha thread, the spacers may also be of adjustable or spring-actionformation. In this way, the two mould halves 11, 12 can be moved towardsone another during the crosslinking process to balance out leakage, byadjusting the spacers (indicated symbolically by the arrow 19 c showingthe direction of rotation) or against a spring action. Of course, themould can be opened and closed in the usual manner, for example by meansof a closure unit which is indicated here only by the arrow symbol 1 a.Adjustment of the gap between the two mould halves 11, 12 to balance outleakage, may also be effected e.g. using this external closure unit.

It is also conceivable that, instead of the continuous annular gap 16and the spacers 19 a and 19 b, a series of segmentous gaps may beprovided, the intermediate areas between the individual segment gapstaking over the function of the spacers. Of course, other configurationsof mould halves are also conceivable.

On the mould wall 17 in the area of the annular gap 16, there is a mask21 which is impermeable to the energy form employed, here this is UVlight, (or a mask which at least has poor permeability compared with thepermeability of the mould), and this mask extends right to the mouldcavity 15, and with the exception of the same, screens all the otherparts, hollow spaces or areas of the mould 1 that are in contact with ormay come into contact with the liquid, uncrosslinked, possibly excessmaterial, from the radiated energy. Partial areas of the lens edge aretherefore formed not by a limitation of the material by mould walls, butby a spatial limitation of the radiation or other forms of energytriggering polymerisation or crosslinking.

In the case of UV light, the mask 21 may be preferably a chromium layer,that can be produced by processes known e.g. from photography orUV-lithography. The mask 21 does not necessary have to be fixed; it mayalso be, for example, removable or exchangeable.

As well as this casting mould illustrated in FIG. 1, otherconfigurations are however also possible, for example the embodimentillustrated in FIG. 2. This casting mould similarly consists of twomould halves 11 and 12, each of which has a curved surface 13 and 14,which together define a mould cavity 15. The mould surface 13 of theupper mould half 11 in the drawing is convex and determines the rear andbasic face of the contact lens; this mould half is normally called thefather mould half. Conversely, the mould surface 14 of the other mouldhalf is concave and is called the mother mould half. It defines thefront face of the contact lens to be manufactured. The edge region ofthe contact lens is defined by the limiting area 22. After filling thelens material into the mother mould half 12, the mould is closed byplacing the male mould half 11 thereon. Normally, a surplus of startingmaterial is used, so that, when the mould is closed, the excess amountis expelled into an overflow area outwardly adjacent to the mould cavity15. The subsequent polymerisation or crosslinking of the startingmaterial takes place by radiation with UV light, or by heat action, orby another non-thermal method. Both the starting material in the mouldcavity 15 and the excess material in the overflow area are therebyhardened. The geometry of the edge of the contact lens is defined by thecontour of the two mould halves 11, 12 in the area in which they touchone another. In order to obtain error-free separation of the contactlens from the excess material, a good seal or expulsion of the excessmaterial must be achieved in the contact zone 22 of the two mould halves11, 12. Only in this way can error-free contact lens edges be obtained.

In general, the oxygen permeability coefficient P of a polymer may bedescribed by the following formula:P=D×S,with

-   -   P: oxygen permeability of the polymer    -   D: oxygen diffusion constant of the polymer    -   S: oxygen solubility in the polymer

The permeability coefficient P is a measure of the degree of oxygenwhich acts on the lens material during the hardening process and thuseffects an inhibition of the polymerisation reaction. The lower theoxygen permeability coefficient, the less oxygen that reaches thesurface of the mould cavity 15 and the lower the inhibition ofpolymerisation at the surface of the lens.

Since, in the embodiments shown here, the UV radiation only comes fromone side, from the top, in fact only the father mould half 11 needs tobe permeable to UV light. If radiation comes from below through themother mould half 12, of course the same applies in reverse. Accordingto an especially suitable and advantageous arrangement of the invention,the mould half which is exposed to UV light is made of quartz, while theother mould half is made of a polymer. However, within the scope of theinvention, both mould halves 11, 12 may also be made of a polymer, andit is also conceivable that the impacting of the crosslinkable materiallocated in the mould cavity 15 with energy that effects crosslinking,may also be effected not only from one side, but also from two sides. Inthis case, it must be ensured that both mould halves are permeable to UVlight.

According to the invention, the materials that may be considered for thecasting moulds are a number of polymers, which however in respect oftheir oxygen permeability must be below the value$5 \times 10^{- 11}\frac{\left( {{ml}\quad O_{2}\quad{at}\quad{STP}} \right)({mm})}{\left( {cm}^{2} \right)(s)({torr})}$

Table 1 lists the oxygen permeability P of various polymers, whereby therelative values are taken from the book: “Polymer Handbook”, and havebeen recalculated into absolute values corresponding to the value givenfor polypropylene in Table 1 on page 61 of the book “Polymerpermeability, 1. Polymers and polymerization”, from Elsevier AppliedScience Publishers LTD 1985, Reprinted 1986. The unit of oxygenpermeability is:$\frac{\left( {{ml}\quad O_{2}\quad{at}\quad{STP}} \right)({mm})}{\left( {cm}^{2} \right)(s)({torr})}$

According to the invention, at least one of the two mould halves 11 and12 consists of a polymer that has an oxygen permeability lower than$5 \times 10^{- 11}\frac{\left( {{ml}\quad O_{2}\quad{at}\quad{STP}} \right)({mm})}{\left( {cm}^{2} \right)(s)({torr})}$TABLE 1 Oxygen permeability of various polymers polymer oxygenpermeability polyacrylonitrile (PA) 0.0025 methacrylate/acrylonitrilecopolymer (MAN) 0.051 CR-39 (polymer consisting of allyl diglycol 0.15carbonate monomer) polymethylmethacrylate (PMMA) 1.4 polypropylene (PP)21 polystyrene (PS) 32

The table shows that the permeability coefficient for polyacrylonitrile(PA) and polymethyl-methacrylate (PMMA) is 10 to 1000 times lower thanthe values for polypropylene (PP) and polystyrene (PS). It has beenshown that the lenses that were produced with plastic moulds made of PPand PS in a normal ambient atmosphere had a considerably higher degreeof slipperiness than lenses produced with moulds made of PA or PMMA.

According to a further concept of the invention, it is advantageous toselect a material that has a high absorption coefficient for UV light,or to provide materials with UV absorbers, in order to attain thedesired UV absorption. The UV stability of the casting moulds and thustheir service life may thereby be decisively improved.

Further studies were carried out with CP-75 UVA and CP-71 UVA (both PMMAwith a UV absorber), PMMA GS-222 from the company Rohm, Goldflex (GF),Delrin (DR), Luran 368R (styrene/acrylonitrile copolymer, SAN), TerluxKR-2812 (methylmethacrylate/acrylonitrile/butadiene/styrene polymer,MABS), Barex 210 (methacrylate/acrylonitrile polymer, MAN), Topas fromthe company Ticono, as well as CR-39 (chemically crosslinked polymerconsisting of allyl diglycol carbonate monomer), which were similarlynotable for their lowoxygen permeability coefficient. It is demonstratedthat very good results are obtained as regards shape and UV stability ofthe moulds, as well as good lens quality, especially using materialsCP-75, CP-71, CR-39, PMMA GS-22 and Barex.

To crosslink the starting material, UV light with a wavelength of λ>280nm is preferably used. By using longer wavelength UV light, the demandsplaced on the UV stability of the polymers are substantially lowered.

It has been shown that when choosing a polymer for the mother mould half12, which has high UV absorption, the quality of the edge of a lensproduced in a device according to FIG. 1 can be improved, compared witha device in which both mould halves consist of quartz/glass.

In the embodiment illustrated in FIG. 1, radiation with UV light tocrosslink the polymer takes place through the male casting mould 11; Ifthe female mould half 12 consists of a material that does not absorb UVlight, reflections of the UV light can occur. The reflected UV light nowcrosslinks the lens material below the mask 21 of the male casting mould11, whereupon a non-sharp contact lens edge is produced, since thematerial around the actual edge is crosslinked by reflected UV light.If, on the other hand, UV-absorbing materials are used for the femalemould half 12, these problems do not arise.

The processes which may be considered for the production of plasticmoulds may be a number of techniques, for example injection moulding,lathing and polishing. These technologies are established, so that theproduction process is relatively simply to carry out and they do notrequire particularly great resources.

In addition, almost all of the above-listed plastics enable opticalcontrol of the lenses taking place through the casting mould, since theyare transparent and also do not become hazy during long-term exposure toUV light.

1. A process for the production of contact lens, comprising polymerizingand/or crosslinking a starting material in a mold to form the contactlens, wherein the starting material is polymerizable and/orcrosslinkable upon irradiation with appropriate energy, wherein the moldhas a first mold half having a first optical surface in contact with thestarting material and a second mold half having a second optical surfacein contact with starting material, wherein said first mold half and saidsecond optical surface in contact with starting material, wherein saidfirst mold half and said second mold half are configured to receive eachother such that the cavity is formed between said first optical surfaceand said second optical surface, wherein at least one of the first andsecond mold halves comprises a polymer having a very low oxygenpermeability characterized by being less than$5 \times 10^{- 11}\frac{\left( {{ml}\quad O_{2}\quad{at}\quad{STP}} \right)({mm})}{\left( {cm}^{2} \right)(s)({torr})}$so that oxygen is prevented from reaching the first and/or secondoptical surfaces during polymerizing and/or crosslinking process andinhibitition of polymerizing and/or crosslinking of the startingmaterial at starting material/optical surface interface is substantiallyminimized or eliminated, and wherein the polymer is selected from thegroup consisting of Terlux KR-2812(methamethacrylate/acrylonitrile/butadiene/styrene copolymer), Barex 210(methacrylate/acrylonitrile copolymer), Ticona's Topas® (cyclic olefincopolymers), and CR-39 (diallyl diglycol carbonate polymer).
 2. Aprocess according to claim 1, wherein the first mold half comprises thepolymer having an oxygen permeability of less than$5 \times 10^{- 11}\frac{\left( {{ml}\quad O_{2}\quad{at}\quad{STP}} \right)({mm})}{\left( {cm}^{2} \right)(s)({torr})}$3. A process according to claim 1, wherein both the first and secondmold halves comprises the polymer having an oxygen permeability of lessthan$5 \times 10^{- 11}\frac{\left( {{ml}\quad O_{2}\quad{at}\quad{STP}} \right)({mm})}{\left( {cm}^{2} \right)(s)({torr})}$4. A process according to claim 3, wherein the energy is UV radiation,and wherein one of the first and second mold halves comprises UVabsorbers distributed in the polymer.
 5. A process according to claim 2,wherein the polymer is Ticona's Topas® (cyclic olefin copolymers).
 6. Aprocess according to claim 5, wherein the first mold half is a male moldhalf.
 7. A process according to claim 5, wherein the first mold half isa female mold half.
 8. A process according to claim 7, wherein theenergy is UV radiation. 9-15. (Canceled)
 16. A process according toclaim 6, wherein the energy is UV radiation.
 17. A plastic mold half formaking contact lenses, comprising an optical surface which is in contactwith a lens material for making contact lenses and defines one of thetwo opposite surfaces of a contact lens to be molded, wherein the lensmaterial is polymerizable and/or crosslinkable upon irradiation with anappropriate energy, and being made from a mold material comprising apolymer selected from the group consisting of Terlux KR-2812(methamethacrylate/acrylonitrile/butadiene/styrene copolymer), Barex 210(methacrylate/acrylonitrile copolymer), Ticona's Topas® (cyclic olefincopolymers), CR-39 (chemically crosslinked polymer consisting of allyldiglycol carbonate monomer), wherein the polymer has a very low oxygenpermeability characterized by a value of less than$5 \times 10^{- 11}\frac{\left( {{ml}\quad O_{2}\quad{at}\quad{STP}} \right)({mm})}{\left( {cm}^{2} \right)(s)({torr})}$so as to be capable of substantially minimize and/or eliminateinhibition of polymerizing and/or crosslinking of the lens material atinterface between the lens material and the optical surface bypreventing oxygen from reaching the optical surface during polymerizingand/or crosslinking process.
 18. The plastic mold half of claim 17,wherein the mold half is a male mold half.
 19. The plastic mold half ofclaim 18, wherein the polymer is Ticona's Topas® (cyclic olefincopolymers).
 20. The plastic mold half of claim 17, wherein the moldhalf is a female mold half.
 21. The plastic mold half of claim 20,wherein the polymer is Ticona's Topas® (cyclic olefin copolymers).
 22. Aplastic mold for making a contact lens, comprising a first mold halfhaving a first optical surface in contact with the lens material and asecond mold half having a second optical surface in contact with lensmaterial, wherein said first mold half and said second mold half areconfigured to receive each other such that the cavity is formed betweensaid first optical surface and said second optical surface, wherein thecavity defines the shape of a contact lens to be molded, wherein thelens material is polymerizable and/or crosslinkable upon irradiationwith an appropriate energy, wherein at least one of the mold halves ismade from a mold material comprising a polymer selected from the groupconsisting of Terlux KR-2812(methamethacrylate/acrylonitrile/butadiene/styrene copolymer), Barex 210(methacrylate/acrylonitrile copolymer), Ticona's Topas® (cyclic olefincopolymers), CR-39 (chemically crosslinked polymer consisting of allyldiglycol carbonate monomer), wherein the polymer has a very low oxygenpermeability characterized by a value of less than$5 \times 10^{- 11}\frac{\left( {{ml}\quad O_{2}\quad{at}\quad{STP}} \right)({mm})}{\left( {cm}^{2} \right)(s)({torr})}$so as to be capable of substantially minimizing and/or eliminatinginhibition of polymerizing and/or crosslinking of the lens material atinterface between the lens material and the first or second opticalsurface by preventing oxygen from reaching the first or second opticalsurface during polymerizing and/or crosslinking process.
 23. The plasticmold of claim 22, wherein the first mold half comprises the polymerhaving an oxygen permeability of less than$5 \times 10^{- 11}{\frac{\left( {{ml}\quad O_{2}\quad{at}\quad{STP}} \right)({mm})}{\left( {cm}^{2} \right)(s)({torr})}.}$24. The plastic mold of claim 23, wherein the polymer is Ticona's Topas®(cyclic olefin copolymers).
 25. The plastic mold of claim 24, whereinthe first mold half is a male mold half.
 26. The plastic mold of claim24, wherein the first mold half is a female mold half.
 27. The plasticmold of claim 23, wherein both the first and second mold halves are madefrom a mold material comprising Ticona's Topas® (cyclic olefincopolymers).